1. Field of the Invention
The invention relates to an electric motor and an electric power steering apparatus employing the electric motor and, more particularly, to the structure of a stator of the electric motor.
2. Description of the Related Art
Electric motors in which a plurality of permanent magnets are arranged on an outer peripheral surface of a rotor provided for rotation in a stator are described in, for example, Japanese Patent Application Laid-Open Nos. HEI 2-111238 and HEI 4-69040. An electric motor as described in these laid-open patent applications is designed to reduce cogging. Cogging is known as the nonuniform variations in torque that occur depending on the construction of a stator, a rotor, a permanent magnet, or the like, even when the motor is not electrified.
In the electric motor as described in the aforementioned laid-open patent applications, each permanent magnet disposed on the outer peripheral surface of the rotor has a generally semicylindrical or "C" shape in a section taken on a plane of rotation of the rotor, and each permanent magnet is disposed on an arc-shaped outer peripheral surface of the rotor that is arched radially outwards. In some cases, therefore, magnetic flux between adjacent permanent magnets locally concentrates in the rotor.
If magnetic flux locally concentrates in the rotor, a great magnetic loss, otherwise known as hysteresis loss, occurs. Moreover, the concentration of local magnetic flux in the rotor promotes the occurrence of a minor loop of magnetic flux in the permanent magnets. The minor loop is a closed loop of magnetic flux extending in a space portion such that the north N and south S poles of the permanent magnets are short-circuited. The resulting magnetic loss in the rotor increases the magnetic resistance in the rotor so that the motor efficiency deteriorates and the output torque decreases. The minor loop of the magnetic flux of the permanent magnets does not contribute to the motor output and, moreover, deteriorates the motor efficiency, thereby reducing the output torque. If a minor loop protrudes into an air gap portion between the rotor and the stator, the minor loop affects the distribution of other magnetic fluxes, resulting in the promotion of the occurrence of torque ripple and cogging. Torque ripple is known as the variations in torque that occur during motor electrification.
The output torque of an electric motor can be increased by increasing the size of each permanent magnet. However, since this requires a large quantity of expensive permanent magnets, the production cost considerably increases as does the size of the electric motor.